Off to med school...and he's already published

LookalikesMale and female tree swallow nestlings are indistinguishable until they’re about a year old. To tell males from females, Andrew Bouland used the polymerase chain reaction to examine the sex chromosomes from blood samples taken in the field.
Photo by Dan Cristol

First authorAndrew Bouland '12 is the first author on a paper in the world's premier ornithological journal. It's the first demonstration that mercury contamination affects the sex ratio of offspring.
Photo by Joseph McClain

Male bluebirdOther species in the study can be easily sexed in the field, even at a young age. For instance, compare the color of this young male bluebird with the wing of the female in the next photo.
Photo by Dan Cristol

Female bluebirdCompared to her brother in the previous photo, the female bluebird's wing shows only a hint of blue.
Photo by Dan Cristol

Andrew Bouland ’12 will enter medical school in a few weeks
with a published scientific paper under his belt.

Several of his med school classmates at the University of
Maryland will no doubt be able to say the same thing, but Bouland’s paper is in
the world’s premier ornithology journal—and he’s the first author. Plus, the
research is groundbreaking.

“This is the first demonstration in any species of wildlife
that mercury affects sex ratio of offspring,” said Dan Cristol, professor of biology
at William & Mary. Cristol is one of the co-authors on Bouland’s paper,
which documents a trend among birds in a mercury-contaminated habitat to have
more female offspring.

The paper, published in the Journal of Avian Biology, is the culmination of research that began
in 2005 in Virginia’s South River. Cristol and his students examined the
effects of mercury on three species of birds—kingfishers, bluebirds and tree
swallows—living in an area of the Shenandoah Valley that suffered industrial
mercury pollution in the 1930s and 1940s.

The paper, titled “Female-biased offspring sex ratios in
birds at a mercury-contaminated river,” shows that all three species were
producing females at a higher rate than expected. Bouland explained that the
three species were selected based on their feeding habits. Kingfishers, which
eat fish directly from the river, showed the highest levels of mercury. Bluebirds,
which have a diet of terrestrial insects, showed the least. Tree swallows,
eating essentially 50 percent aquatic insects and 50 percent terrestrial, were
intermediate in terms of mercury levels in their blood.

Bouland points out that even though each species of bird had
different levels of mercury contamination, all three species produced more
female offspring than expected.

“Species didn’t have an effect on how skewed nestling ratios
were in terms of production of females. That’s an interesting finding in
itself,” he said.

There are two hypotheses to explain the mechanism that makes
mercury-contaminated birds tend to have daughters at a higher rate. The first,
Bouland explained, is that mercury could somehow disrupt the dispersal of
androgens, hormones generally associated with development of male offspring.

The second hypothesis, he said, is that mostly female
clutches could be an adaptive, evolutionary response by the mothers weakened by
mercury. Either way, an overabundance of daughters is bad news for birds.

“The implication for this is in terms of effective
population sizes of these birds. You’re going to see more females than males,”
he said. “This is a problem because these birds are mostly monogamous. So when
you have more females than males, that poses a problem for the population.”

The researchers in the field, which included co-authors
Ariel White, Kevin Lonabaugh and Claire Varian-Ramos, collected blood from
nestlings and mothers. Bouland noted that mothers were sampled because unlike
humans, the female is the heterogametic gender in birds, controlling the sex of
the offspring.

The researchers were able to easily sex the kingfishers and
bluebirds in the field, using feather color to distinguish male from female. Young
tree swallows are impossible to sex by sight, so Bouland determined the genders
in the lab, using the polymerase chain reaction to examine the sex chromosomes
of the samples.

The findings are important for studies of wildlife in
contaminated areas, but also have immediate human implications, Cristol said.

“For one thing, this shows that it’s important to understand
other species to try to predict environmental threats to humans better,” he
said. “Interestingly, this skewed ratio toward female offspring has already
been shown to be the case in humans exposed to mercury.”

Bouland explained that human births in Japan among the
population affected by the enormous Minamata Bay methylmercury contamination
showed a similar skewing toward females. There have also been several studies
of Arctic villages in which very few boys have been born recently. Cristol
points out that humans who ingest mercury through eating wildlife, such as the
indigenous people in the Arctic, are also exposed to many other chemicals, “so
it’s not as clear a case as with our birds.”